The present invention relates to a process for polymerizing olefins and in particular to a process for the polymerization of olefins using a supported catalyst composition comprising a transition metal compounds and an activator comprising an aluminoxane and an organoboron compound. The process is particularly suitable for operation in the gas phase and preferred transition metal compounds are metallocene complexes.
The process according to the present invention is also particularly suitable for the preparation in the gas phase of copolymers having an improved melt strength.
In recent years there have been many advances in the production of polyolefin copolymers due to the introduction of transition metal compounds and in particular metallocene catalysts. Metallocene catalysts offer the advantage of generally higher activity than traditional Ziegler catalysts and are usually described as catalysts which are single-site in nature. Because of their single-site nature the polyolefin copolymers produced by metallocene catalysts often are quite uniform in their molecular structure. For example, in comparison to traditional Ziegler produced materials, they have relatively narrow molecular weight distributions (MWD) and narrow Short Chain Branching Distribution (SCBD). Although certain properties of metallocene products are enhanced by narrow MWD, difficulties are often encountered in the processing of these materials into useful articles and films relative to Ziegler produced materials. In addition, the uniform nature of the SCBD of metallocene produced materials does not readily permit certain structures to be obtained.
The use of these metal complex based olefin polymerisation catalysts is now well established. Typically the metallocene complex comprises a bis(cyclopentadienyl)zirconium complex for example bis(cyclopentadienyl)zirconium dichloride or bis(tetramethylcyclopentadienyl)zirconium dichloride. Examples of such complexes may be found in EP 129368, EP 206794, and EP 260130.
In such catalyst systems the metal complex is used in the presence of a suitable activator. The activators most suitably used with such metallocene complexes are aluminoxanes, most suitably methyl aluminoxane (MAO). Other suitable activators are boron compounds, in particular perfluorinated boron compounds.
More recently complexes having a single or mono cyclopentadienyl ring have been developed. Such complexes have been referred to as ‘constrained geometry’ complexes and examples of these complexes may be found in EP 416815 or EP 420436. In such complexes the metal atom eg. zirconium or titanium is in the highest oxidation state.
Other complexes however have been more recently developed in which the metal atom may be in a reduced oxidation state. Examples of both the bis(cyclopentadienyl) and mono(cyclopentadienyl) complexes have been described in WO 96/04290 and WO 95/00526 respectively.
The above monocyclopentadienyl metallocene complexes are utilised for polymerisation in the presence of a cocatalyst or activator. Typically activators are aluminoxanes, in particular methyl aluminoxane or compounds based on boron compounds. Examples of the latter are boranes, for example tris(pentafluorophenyl)borane, or borates such as trialkyl-substituted ammonium tetraphenyl- or tetrafluorophenyl-borates. Catalyst systems incorporating such borate activators are described in EP 561479, EP 418044 and EP 551277.
When used for the gas phase polymerisation of olefins, metallocene complexes may typically be supported for example on an inorganic oxide such as silica. Such supports may be typically dehydrated by calcining before use or may be pretreated with an organoaluminium compound to passivate the surface of the silica.
Numerous references have described the use of supported metallocene catalyst compositions for use in the gas phase polymerisation of olefins. A number have also described the combination of more than one activator with such supported metallocene complexes.
The aforementioned WO 95/00526 describes in general terms the use of the reduced oxidation state monocyclopentadienyl complexes with both aluminoxane and borane activators. Although supported catalyst systems are described, all the examples of gas phase polymerisations with supported catalysts are limited to the use of a single activator, typically tris(pentafluorophenyl)borane.
WO 99/15534 describes the combination of aluminoxanes with a fluoroaryl ligand source such as tris(pentafluorophenyl)borane as activators for metallocene complexes. When supported the activator combinations may be used for the gas phase polymerisation of olefins. The gas phase examples in the reference utilise silica supports that have been calcined prior to contact with the activator components.
WO 00/15672 describes functionalized catalyst supports having chemically bonded aluminium containing groups prepared by the combination of supports having reactive functional groups with a source of aluminium. Where exemplified, in the gas phase, the use of aluminoxane/borane activator combinations are supported on silica which has been dehydrated.